Wide band gap III-N FETs provide significant advantages over silicon super junction MOSFETs, offering lower gate charge, faster switching speeds, and smaller reverse recover charge. High speed III-N switches can exhibit in-circuit switching speeds in excess of 200V/ns, compared to current silicon technology at less than 50V/ns, with an accompanying high di/dt during switching. Using high-speed III-N power switches involves balancing requirements for heat transfer, ease of assembly, and high-speed, low-inductance electrical interconnection. Conventional leaded power packages, such as any of the variations of the TO-220 package, can be used with III-N power switches. The combination of a metal mounting tab and flexible copper leads permits attachment of the package to effective heat sinks in a variety of configurations. Connection to a PCB with conventional soldering techniques permits ease of manufacture.
Nonetheless, the package leads typically introduce undesirable inductance. Reduction in switching speed caused by this inductance may be an acceptable design compromise, but instability may still present a problem. Since a power switch can be a high-gain device, if allowed to operate in a linear mode, care should be taken that any oscillations due to parasitic resonances do not couple to a node where positive feedback may sustain or amplify the oscillations.
FIG. 1 is a circuit diagram of a half bridge circuit comprising a gate driver 102, a high side III-N transistor 104 coupled to a high voltage node 106, and a low side III-N transistor 108 coupled to a ground node 110. Two terminals of the gate driver 102 are coupled to respective gates of the transistors 104 and 108, and two terminals of the gate driver are coupled to respective sources of the transistors 104 and 108, such that the gate driver is able to apply voltage signals to the gates of each of transistors 104 and 108 relative to their respective sources. An inductive load 114 is coupled to the half bridge circuit at a load node 112.
In operation, the gate driver 102 can operate the transistors 104 and 108 in a constant-current mode (CCM), switching rated current at rated voltage. For example, the high voltage node 106 can provide a voltage of 400V or 600V or greater, and the III-N transistors can be configured with a rating to withstand the resulting high currents. Due to the inductance of the load 114, current flowing through the load 114 cannot change instantaneously.
To illustrate the operation of the half bridge, consider an example scenario where the gate driver 102 turns the high side transistor 104 on and turns the low side transistor 108 off. Current flows from the high voltage node 106, through the high side transistor 104, and through the load node 112 to the load 114. When the gate driver 102 turns the high side transistor 104 off, the inductance of the load 114 drives the voltage at the load node 112 negative, which allows current to flow up through the low side transistor 108 even though it is off. If the half bridge is implemented using a conventional package, the undesirable inductance introduced by the package leads can cause significant ringing and oscillation related to transient current flowing through the circuit, which can interfere with a stable, efficient switching function. In order avoid sustained oscillation, it is important to guarantee that the gate stays turned off when the complementary device is turning on and any potential oscillation is removed from the circuit.
One method to minimize the oscillation is to optimize the PCB layout to minimize the gate-drain capacitance (CGD), which strongly affects parasitic turn-on due to the voltage slew rate dv/dt. Alternative solutions are described below.